Parts for fluid

ABSTRACT

A specified metal member included among the components of a valve, coupling or like fluid handling part for use in piping and fluid control devices is made of an alloy comprising, in % by weight, 0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03% of P, up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25% of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and the balance substantially Fe and other inevitable impurities, the alloy having a CRI (crevice corrosion resistance index) value in the range of 40≦CRI≦55, as determined from the expression CRI=[Cr]+4×[Mo]+30×[N] wherein the amount of alloy components present in combination in the alloy to ensure crevice corrosion resistance are expressed in % by weight.

TECHNICAL FIELD

The present invention relates to fluid handling parts, such as couplingsfor use in piping systems and valves or like fluid control devices foruse in fluid control apparatus, and more particularly to fluid handlingparts suitable for use in apparatus for manufacturing pharmaceuticals orfoods.

BACKGROUND ART

Couplings, valves, and the like (to be referred to collectively as“fluid handing parts”) are generally used as parts for universal use influid control apparatus and various piping systems.

For example, reactors for use in manufacturing chemicals or foods andtanks or the like for use in storing or transporting chemicals or foodsare provided with metal valves for feeding, transporting or deliveringthe material or product. Carbon steel, low alloy steel, stainless steel,Ni-base alloy, titanium, Ni/Cu alloy or the like is selectively used formetal valves depending on the components, purity and temperature of thechemical or food. SUS304 steel, SUS316L steel or like stainless steel,which is generally considered to be highly resistant to corrosion, iswidely used especially for metal valves for use in manufacturing,storing or transporting materials and products of various chemicals orfoods. However, many of corrosion defects or faults caused to metalvalves are almost all attributable to crevice corrosion due to chlorideions contained in chemicals or foods. Further in many cases, it iscommon practice to sterilize or wash with water the interior of valvesfor maintenance to prevent the development or growth of crevicecorrosion of the valve or microorganisms therein, but if the step ofwashing or sterilization and the step of drying are imperfect, chlorideions frequently remain on the inner surfaces of the valve or in crevicesin the structure thereof. Accordingly, the flange as tightened up on thevalve body or the packing portion of the body is susceptible to crevicecorrosion, consequently necessitating much time and labor for repairsand permitting metal ions released by crevice corrosion to degrade thequality of the product, hence a very serious problem.

If an attempt is made to improve the corrosion resistance of SUS316members so as not to give rise to the problem of corrosion when they areused in food or pharmaceutical manufacturing apparatus wherein solutionsof high salt concentration are used, another problem arises anew such asa reduction in strength or in hardness, so that it has been nearlyimpossible to ameliorate the crevice corrosion resistance only ofconventional fluid handling parts conforming to the requiredspecifications while permitting the part to retain the othercharacteristics.

An object of the present invention is to provide a fluid handling partwhich is improved in crevice corrosion resistance while retaining theother properties.

DISCLOSURE OF THE INVENTION

The present invention provides a fluid handling part which is free ofthe foregoing problems. From the above viewpoint, we have conductedresearch on the environment for chemicals and foods to which valves,couplings and like fluid handling parts are exposed to develop crevicecorrosion and consequently found that crevice corrosion damage or faultsare attributable in many cases to the presence of no small amounts ofchloride ions in chemicals and foods. We have further found that thecorrosive environment for use in evaluating crevice corrosion can besimulated by a seawater environment, and tested various stainless steelvalves for exposure in natural seawater in view of the above findings.More specifically, natural seawater having the normal temperature wascontinuously passed through valves for about six months to check thevalves for corrosion and determine whether the exposure to seawatercaused crevice corrosion faults. Thus, we have conducted intensiveresearch and identified specific fluid channel opening-closing and fluidcontrol valves for use in manufacturing, storage and transport, andspecific pipe couplings for use in manufacturing, storage and transportto accomplish the present invention. To sum up, the present inventionhas the following features.

The invention provides a valve, coupling or like fluid handling part foruse in piping and fluid control devices which is composed of a pluralityof components, the plurality of components including a specified metalmember having a contact surface to be brought into contact with one ofthe other components, the contact surface having an end exposed on anexterior of the fluid handling part, the fluid handling part beingcharacterized in that the metal member is made of an alloy comprising,in % by weight, 0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, upto 0.03% of P, up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr,15 to 25% of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N,and the balance substantially Fe and other inevitable impurities, thealloy having a CRI (crevice corrosion resistance index) value in therange of 40≦CRI≦55, as determined from the expression:CRI=[Cr]+4×[Mo]+30×[N]wherein the amounts of alloy components present in combination in thealloy to ensure crevice corrosion resistance are expressed in % byweight.

Preferably, the alloy further comprises up to 2% of W and/or up to 2% ofV.

The fluid handling part of the invention is composed of a plurality ofcomponents which include a specified metal member having a contactsurface to be brought into contact with one of the other components, thecontact surface having an end exposed on the exterior of the fluidhandling part. The specified metal member, i.e., the member susceptibleto crevice corrosion, is made of an alloy comprising, in % by weight,0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03% of P,up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25% of Ni,4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and the balancesubstantially Fe and other inevitable impurities. The alloy has a CRIvalue in the range of 40≦CRI≦55, as calculated from the expressionCRI=[Cr]+4×[Mo]+30×[N] wherein the amounts of alloy components presentin combination in the alloy to ensure crevice corrosion resistance areexpressed in % by weight. Accordingly, the part can be improved increvice corrosion resistance without entailing a reduction in strengthand in hardness which has been a disadvantage resulting from theimprovement in crevice corrosion resistance.

The metal member is preferably up to 1, more preferably up to 0.2, insurface roughness Ra.

Although couplings, valves, etc. are exemplified as fluid handlingparts, these examples are not limitative; the invention can be embodiedalso as various other parts.

For example, the fluid handling part is a valve comprising a body, anactuator and a threaded member, and at least one of the body, theactuator and the threaded member is the specified metal member (made ofthe above alloy). Alternatively the fluid handling part is a diaphragmvalve wherein a nonmetallic diaphragm is held between a metal body and ametal bonnet, and each of the body and the bonnet is the specified metalmember (made of the above alloy). In the latter case, the portion wherethe diaphragm is held between the body and the bonnet is verysusceptible to crevice corrosion. By giving improved crevice corrosionresistance to the portion, remarkably improved durability can be givento the diaphragm valve in its entirety.

Further alternatively, the fluid handling part is a pipe coupling to beassembled by tightening up a cap nut as screwed on an externallythreaded portion provided on an outer periphery of a tubular couplingmember, and at least one of the coupling member and the cap nut is thespecified metal member (made of the alloy).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a valve as a first embodiment offluid handling part of the invention.

FIG. 2 is a sectional view showing a diaphragm value in a channelclosing state, as a second embodiment of fluid handling part of theinvention.

FIG. 3 is a view in vertical section showing the valve of FIG. 2 in achannel opening state.

FIG. 4 is an exploded view in longitudinal section showing a pipecoupling as a third embodiment of fluid handling part of the invention.

FIG. 5 is a view in longitudinal section showing the pipe coupling ofFIG. 4 as assembled.

FIG. 6 is an exploded view in vertical section showing a pipe couplingas a fourth embodiment of fluid handling part of the invention.

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings. However, the invention is not limited to thedrawings showing the embodiments.

FIG. 1 shows a first embodiment of fluid handling part of the invention.In the description of the first embodiment, the left- and right-handsides of the drawing will be referred to as “left” and “right,”respectively.

The fluid handling part of this embodiment is a needle stop valve, whichcomprises a bottomed hollow cylindrical body 2 provided at a lowerportion thereof with a tubular leftward projection 8 and a tubularrightward projection 9, a solid cylindrical stem 3 inserted in the body2 upwardly and downwardly movably, a hollow cylindrical guide 4 fittedin an upper portion of the body 2 for guiding the upward and downwardmovement of the stem 3, a panel nut 5 screwed on the lower end of anexternally threaded portion 10 formed on the outer periphery of upperpart of the body 2, a cap nut 6 screwed on the top part of the threadedportion 10, and a handle 7 provided on the upper end of the stem 3.

The stem 3, guide 4, cap nut 6 and handle 7 constitute an actuator ofthe fluid handling part 1.

The body 2 is provided at the lower portion thereof with a fluid inletchannel 2 a extending from a position close to the center of the bodyslightly upwardly leftward and communicating with an inside channel ofthe leftward projection 8, a fluid outlet channel 2 b extending from aposition above the end, toward the center, of the inlet channel 2 aslightly downwardly rightward and communicating with an inside channelof the rightward projection 9, and a communication channel 2 c extendingvertically to hold the two channels 2 a, 2 b in communication. Thecommunication channel 2 c is stepped to give a smaller diameter to thelower portion thereof than to the upper portion thereof. The body 2 hasan inner peripheral surface positioned above the communication channel 2c and defining a stem guide passage 11 extending vertically. The stemguide passage 11 has an internally threaded lower portion 11 a slightlyextending into the communication channel 2 c. The guide passage 11 hasan upper portion having a larger diameter than the threaded portion 11 aand having the hollow cylindrical guide 4 fitted therein.

The stem 23 has a conical lower end portion 3 a tapered toward anextremity thereof, and an externally threaded portion 3 b positionedabove the conical portion 3 a, having a larger diameter than the otherportion and screwed in the internally threaded portion 11 a of the body2.

The guide 4 bears on a stepped portion at the upper end of theinternally threaded portion 11 a so as to cause the upper end of theguide to project beyond the upper end face of the body. The top wall ofthe cap nut 6 has a through hole for an upper end portion of the stem 3to extend therethrough. The cap nut 6 is screwed on the externallythreaded portion 10 of the body 2, whereby the guide 4 is fixed to thebody 2. The upper end of the stem 3 extends upward beyond the cap nut 6and has the handle 7 attached thereto.

The leftward projection 8 and the rightward projection 9 both tubularand positioned at the lower portion of the body 2 are each provided witha pipe coupling portion. Each of the projections 8, 9 is provided with afront ring 12 and a back ring 13 which are fitted around a pipeprojecting from the projection, and with a cap nut 14 for tightening upthe rings 12, 13 to fix the pipe to the projection 8 (9).

As to the materials of the members 2, 3, 4, 5, 6, 7, the guide 4 is madeof PTFE and PFA, the handle 7 is made of ADC12, and the body 2, stem 3,panel nut 5 and cap nut 6 are made of an alloy comprising, in % byweight, 0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03%of P, up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25%of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and thebalance substantially Fe and other inevitable impurities. The alloy hasa CRI value in the range of 40≦CRI≦55, as calcularted from theexpression CRI=[Cr]+4×[Mo]+30×[N] wherein the amounts of alloycomponents present in combination in the alloy to ensure crevicecorrosion resistance are expressed in % by weight.

The portions where the body 2 is in contact with the ends of the nuts 5,6, and the portion where the stem 3 is in contact with the hole-defininginner periphery of the top wall of the cap nut 6 are susceptible tocrevice corrosion. The metal members 2, 3, 5, 6, which are prone tocrevice corrosion while in use, are made of the alloy described above.Furthermore, the contact portions of the metal members 2, 3, 5, 6 arefinished to a surface roughness of about 0.1 in Ra. The surfaceroughness and the use of the stainless steel of the above compositionproduce a synergistic effect to give further improved crevice corrosionresistance.

It is also desirable to use the above alloy for the back ring 13 and thecap nut 14 of the pipe coupling provided on each of the tubular leftwardand rightward projections 8, 9 at the lower portion of the body 2 toensure improved crevice corrosion resistance.

FIGS. 2 and 3 show a second embodiment of fluid handling part of theinvention. FIG. 2 shows the part in a channel closing state, and FIG. 3the same in a channel opening state.

The fluid handling part 21 of this embodiment is a diaphragm valve,which comprises a body 22 having a fluid inlet channel 22 a and a fluidoutlet channel 22 b, a diaphragm 23 for opening or closing the channels22 a, 22 b of the body 22, and an actuator 24 for moving the diaphragm23 to an opening position or to a closing position.

The body 22 has at its center a circular cavity 22 c facing upward. Theinlet channel 22 a and the outlet channel 22 b have respective innerends which are opened to this cavity 22 c.

The diaphragm 23 comprises a square diaphragm 23 a having embeddedtherein one end of a suspending member 23 c, and a circular diaphragm 23b provided in intimate contact with the upper surface of the squarediaphragm 23 a. The diaphragm 23 is so shaped as to project downward atits center. In the channel closing state shown in FIG. 2, the diaphragmcentral portion is in bearing contact with the bottom surface of thebody 22 defining the cavity 22 c to thereby close a channel extendingfrom the inlet channel 22 a to the outlet channel 22 b. The cavity sideof the body 22 has no projection (wear portion), and the inside channelof the body 22 extends straight.

The actuator 24 comprises a bonnet 25 in the form of a hollow cylinderhaving a top wall, a casing 26 provided over the top wall of the bonnet25, a stem 27 having a lower end connected to the suspending member 23 cand an upper end projecting upward beyond the casing 26, a piston 28disposed inside the casing 26 upwardly or downwardly movably and securedto an intermediate portion of the stem 27, and a diaphragm holder 29provided at the lower end portion of the stem 27. The body 22, thediaphragm 23 and the actuator 24 are fitted to one another so that theouter edge portion of the square diaphragm 23 a of the diaphragm 23 isheld between a flange portion 25 a of the bonnet 25 and an opening edgeportion 22 d of the body 22. The members 22 to 24 are fastened togetherwith unillustrated screw members to assemble the diaphragm valve 21.

When compressed air is introduced into the casing 26 below the piston 28in the channel closing state shown in FIG. 2, the piston 28 and the stem27 move upward, and the diaphragm 23 positioned in the cavity 22 c ofthe body 22 also moves upward to bring the valve into the channelopening state as shown in FIG. 3, whereby a straight channel having noportion permitting stagnation of fluid and extending from the inletchannel 22 a to the outlet channel 22 b is formed inside the body 22. Inthe channel opening state, the diaphragm 23 is elastically deformed sothat an upwardly projecting annular ridge is formed between the centralportion thereof and the diaphragm outer edge portion held between theflange portion 25 a of the bonnet 25 and the opening edge portion 22 dof the body.

As to the materials of the members 22, 23, 25, 26, 27, 28, the piston 28is made of SUSF316L, the square diaphragm 23 a is made of PTFE, thecircular diaphragm 23 b is made of butyl rubber, and the body 22, bonnet25, casing 26 and stem 27 are made of an alloy comprising, in % byweight, 0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03%of P, up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25%of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and thebalance substantially Fe and other inevitable impurities. The alloy hasa CRI value in the range of 40≦CRI≦55, as determined from the expressionCRI=[Cr]+4×[Mo]+30×[N] wherein the amount of alloy components present incombination in the alloy to ensure crevice corrosion resistance areexpressed in % by weight.

The butt joint between the body 22 and the bonnet 25 with the diaphragm23 interposed therebetween, the joint between the bonnet 25 and thecasing 26, and the portion of contact between the stem 27 and the topwall inner periphery of the casing 26 defining a through hole aresusceptible to crevice corrosion, so that the metal members 22, 25, 26,27, which are prone to crevice corrosion while in use, are made of thealloy described above. Furthermore, the contact portions of the metalmembers 22, 25, 26, 27 are finished to a surface roughness of about 0.1in Ra. The surface roughness and the use of the stainless steel of theabove composition produce a synergistic effect to give further improvedcrevice corrosion resistance.

FIGS. 4 and 5 show a third embodiments of fluid handling part of theinvention. In the description of the third embodiment, the left- andright-hand sides of the drawing will be referred to as “front” and“rear,” respectively.

The fluid handling part 30 of this embodiment is a pipe coupling, whichcomprises a tubular body (coupling member) 31 for a pipe 32 to beinserted in through the rear end thereof, a front ring 33 and a backring 34 which are to be fitted around the pipe 32 projecting from therear end of the body 31, and a cap nut 35 for tightening up the frontand back rings 33, 34 to fix the pipe 32 to the body 31.

The body 31 has an outer flange 36 formed around the outer periphery ofan intermediate portion thereof and externally threaded portions 37, 38respectively at front and rear end portions thereof. The inner peripheryof the body 31 has a large-diameter rear-end portion 31 a slightlylarger than the front portion of the body inner periphery. The body 31has a tapered face 31 b formed on the inner periphery of its rear endand tapered toward the front.

The cap nut 35 is provided at the front-end side thereof with aninternally threaded portion 35 a, which is screwed on the externallythreaded rear-end portion 38 of the body 31. The cap nut 35 has an innerflange 35 b at the rear end thereof.

The front ring 33 is provided on its outer periphery with a tapered face33 a shaped in conformity with the shape of the tapered face 31 b at therear end of the body 31. The front ring 33 has an annular recess 33 bformed at the inner periphery of its rear end and tapered toward thefront. The back ring 34 is provided at its front end with an annularprojection 34 a tapered toward the front and fittable into the recess 33b of the front ring 33.

When the cap nut 35 of the pipe coupling 30 is tightened, the front faceof the inner flange 35 b of the cap nut 35 comes into bearing contactwith the rear face of the back ring 34, advancing the back ring 34,whereby the projection 34 a of the back ring 34 is fitted into therecess 33 b of the front ring 33. The front ring 33 is moved forwardalong with the back ring 34 and has its front end brought into contactwith the tapered face 31 b of the body 31. When the cap nut 35 isfurther tightened up, the respective front ends of the front ring andthe back ring 34 are deformed inward, caused to bite into the pipe 31and firmly pressed against the pipe 32.

As to the materials of the members 31, 33, 34, 35, front ring 33 is madeof SUS316, and the body 31, back ring 34 and cap nut 35 are made of analloy comprising, in % by weight, 0.001 to 0.01% of C, up to 5% of Si,up to 2% of Mn, up to 0.03% of P, up to 100 ppm of S, up to 50 ppm of O,18 to 25% of Cr, 15 to 25% of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu,0.1 to 0.3% of N, and the balance substantially Fe and other inevitableimpurities. The alloy has a CRI value in the range of 40≦CRI≦55, ascalculated from the expression CRI=[Cr]+4×[Mo]+30×[N] wherein the amountof alloy components present in combination in the alloy to ensurecrevice corrosion resistance are expressed in % by weight.

The portion of screw-thread engagement between the body 31 and the capnut 35 and the portion of contact between the cap nut 35 and the backring 34 are susceptible to crevice corrosion, so that the metal members31, 34, 35, which are prone to crevice corrosion while in use, are madeof the alloy described.

FIG. 6 shows a fourth embodiment of fluid handling part of theinvention.

The fluid handling part 40 of this embodiment is a pipe joint, whichcomprises first and second tubular coupling members 41, 42 havingrespective fluid channels 41 b, 42 b in communication with each other, agasket 43 interposed between butting end faces of the two couplingmembers 41, 42, and threaded means for connecting the two couplingmembers 41, 42 together. The first and second coupling members 41, 42have respective annular ridges 41 a, 42 a formed on their butting endfaces. The second coupling member 42 has a flange 42 c close to the endthereof. The threaded means comprises an externally threaded portion 46provided on the first coupling member 41, and a cap nut 44 fittingaround the second coupling member 42 and screwed on the externallythreaded portion 46 of the first coupling member 41 so that the innersurface of a top wall of the nut is pressed against the flange 42 c witha thrust ring 45 interposed therebetween.

With this pipe coupling 40, the cap nut 44 screwed on the first couplingmember 41, when tightened up, deforms the gasket 43, whereby a propersealing force is available.

As to the materials for the members 41, 42, 43, 44, the gasket 43 ismade of SUS316, and the first coupling member 41, second coupling member42 and cap nut 44 are made of an alloy comprising, in % by weight, 0.001to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03% of P, up to100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25% of Ni, 4.5to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and the balancesubstantially Fe and other inevitable impurities. The alloy has a CRIvalue in the range of 40≦CRI≦55, as determined from the expressionCRI=[Cr]+4×[Mo]+30×[N] wherein the amount of alloy components present incombination in the alloy to ensure crevice corrosion resistance areexpressed in % by weight.

The portion of screw-thread engagement between the first coupling member41 and the cap nut 44 and the portion of contact between the secondcoupling member 42 and the inner periphery of the cap nut (44) top walldefining a through bore are susceptible to crevice corrosion, so thatthe metal members 41, 42, 44, which are prone to crevice corrosion whenin use, are made of the alloy described.

The present invention gives high durability to fluid handling parts foruse in manufacturing, storing or transporting chemicals or foods againstthe damage or faults that would result from crevice corrosion, bystrictly limiting the chemical components and CRI (crevice corrosionresistance index) value of the part. For this purpose, we have foundthat when the contents of main alloy components of stainless steel foruse in making fluid handling parts, i.e., the Cr, Ni, Mo, Cu, N, W and Vcontents, are limited within restricted ranges so as to give a CRI valuenot lower than the lower limit, outstanding durability can be given tothe material for use in the foregoing environment.

Described below are the requirements as to the composition of the alloy(stainless steel) for making the fluid handling parts of the inventionand reasons for limiting the contents.

[C Content: 0.001-0.01%]

Although C is generally detrimental to the corrosion resistance ofstainless steels, presence of an amount of C is necessary from theviewpoint of strength. A very low C content of less than 0.001% entailsan increased production cost, whereas when in excess of 0.01%, the Ccontent results in greatly impaired corrosion resistance, hence 0.001%to up to 0.01%.

[Si Content: Up to 5%]

Si is an element effective for giving improved oxidation resistance tostainless steels. Presence of more than 5% of Si leads to seriouslyimpaired hot workability. Accordingly, the Si content is limited to nothigher than 5.0%.

[Mn Content: Up to 2%]

Mn is an element for stabilizing austenite, can be used as a substitutefor expensive Ni, but is ineffective for giving corrosion resistance foruse in salt water as contemplated by the present invention, if more than2.0% of Mn is present. The upper limit for the Mn content not affectingcorrosion resistance is 2.0%.

[P Content: Up to 0.03%]

Low P contents are desirable from the viewpoint of corrosion resistanceand hot workability. In excess of 0.03%, seriously impaired hotworkability will result. Accordingly, the P content should be up to0.03%.

[S Content: Up to 100 ppm]

S is an element exerting a marked influence on hot workability as wellas on corrosion resistance. The lower the content, the better. The Scontent is therefore up to 100 ppm (0.01%).

[O Content: Up to 50 ppm]

Like S, O is also an element exerting a marked influence on hotworkability. The lower the content, the better. O is limited to nothigher than 50 ppm which is a value available by the usual stainlesssteel making process.

[Cr Content: at Least 18% to not Higher than 25%]

Cr is a basic component according to the present invention. This elementis added in the presence of Ni, Mo, Cu and N. Presence of at least 18%of Cr is necessary to obtain satisfactory corrosion resistance. Althoughthe corrosion resistance improves with an increase in the Cr content,amounts exceeding 25% present some difficulty in making the alloy andlead to an increased cost. The Cr content should therefore be limited tothe range of at least 18% to not higher than 25%.

[Ni Content: at Least 15% to not Higher than 25%]

Like Cr, Mo, Cu and N, Ni is also a basic component of the stainlesssteel of the invention. To facilitate the production of thick plates ofstainless steel, it is required to give the steel a metal structure ofaustenitic phase and to add Ni. At least 15% of Ni should be present togive the steel of the invention an austenitic phase. The presence of anexcess of Ni not only results in an increased cost but also difficultyin making the steel. To ensure a low cost and austenitic phase, theupper limit for the Ni content is 25%.

[Mo Content: at Least 4.5% to not Higher than 7.0]

Like Cr, Ni, Cu and N, Mo is a basic component of the stainless steel ofthe invention. Mo is essential in obtaining high corrosion resistance ina saltwater environment. Mo content of 4.5% to 7.0% is effective in thepresence of Cr and N. With less than 4.5% of Mo present, insufficientcorrosion resistance will result, whereas in excess of 7.0%, the effectto give improved corrosion resistance levels off, and the production ofthe alloy encounters difficulty and requires a high cost.

[Cu Content: at Least 0.5% to not Higher than 3.0%]

Cu is an essential element for giving high corrosion resistance in asaltwater environment when present conjointly with Cr, Ni, Mo and N.When at least 0.5% of Cu is present, a remarkable effect is available bythe combination of these elements, whereas if more than 3.0% of Cu ispresent, the effect to increase the corrosion resistance levels off andlower hot workability will result. Accordingly, Cu content is limited tothe range of 0.5% to 3.0%.

[N Content: at Least 0.1% to not Higher than 0.3%]

N is used as a basic component in combination with Cr, Ni, Mo and Cu. Nis a highly effective austenite forming element and at the same time, anelement for inhibiting crevice corrosion of stainless steels. At least0.1% of N must be present if stabilized corrosion resistance is to beobtained. Addition of more than 0.3% of N presents extreme difficulty inmaking the stainless steel and impairs the hot workability of the steel.The N content is therefore limited to the range of at least 0.1% to nothigher than 0.3%.

[W Content: Up to 2%]

If W is added conjointly with Cr, Mo, N and V, a further stabilizedpassive film will be formed, giving improved crevice corrosionresistance to the stainless steel in seawater. Up to 2% of W is used inaccordance with the environment because presence of more than 2% of Wentails seriously impaired hot workability.

[V Content: Up to 2%]

If V is added conjointly with Cr, Mo, N and W, a further stabilizedpassive film will be formed, giving improved crevice corrosionresistance to the alloy in seawater. Up to 2% of V is used in accordancewith the environment. The higher the V content, the higher the crevicecorrosion resistance, whereas use of more than 2% of V seriously impairsthe hot workability of the stainless steel, presenting difficulty insteel making and leading to an economical disadvantage. Accordingly, theV content is limited to 2% if highest.

[CRI Value: at Least 40 to not Greater than 55]

When the CRI value calculated from the expression [Cr]+4×[Mo]+30×[N] isat least 40, the stainless steel is substantially free from crevicecorrosion. CRI values over 55 result in a higher steel making cost,posing problems to the universal usefulness of the part or entailing anincreased valve fabricating cost. For these reasons, the CRI value islimited to the range of at least 40 to not greater than 55 from theviewpoint of assuring high crevice corrosion resistance and a costreduction.

In the foregoing first to fourth embodiments, the contact portions ofthe metal members 2, 3, 6, 22, 25, 26, 27, 31, 34, 35, 41, 43, 44 aresurface-finished to a surface roughness of about 0.1 in Ra. This rendersthe contact portions less susceptible to crevice corrosion.

EXAMPLES

The present invention will be further described with reference to thefollowing examples.

For a comparison, Table 1 shows the chemical compositions of fluidhandling parts of the invention and comparative parts, the resultsobtained by passing natural seawater of normal temperature through theparts for about six months and thereafter checking the parts for crevicecorrosion faults and the results of overall evaluation.

The blank materials for the flow handling parts were prepared by theelectric furnace-AOD process or electric furnace-VAC process. The moltensteels obtained were cast into continuous slabs under the usualconditions, followed by a soaking treatment at 1150° C. to 1250° C. for0.5 to 1 hour. After a surface treatment, valves of the constructionshown in FIGS. 2 and 3 were fabricated, subjected to a solution heattreatment and used for testing. The finished surfaces had a surfaceroughness of 0.1 in Ra.

The overall evaluation was conducted with reference to the results ofthe exposure test conducted in natural seawater and the CRI valuesaccording to the following criteria.

-   -   Mark ⊚: valve fulfilling the two requirements of being free of        any crevice corrosion fault and having a CRI value of at least        40 to not greater than 55.    -   Mark X: valve failing to fulfill one or both of the above        requirements.

With reference to Table 1, the valves less than 35 in CRI value were allfound to have developed crevice corrosion. The valve No. 5 with a CRIvalue of 42.93 was low in Ni, Mo and Cu contents. Although increasedonly in Cr content and having a greater CRI value, this valve shows thatcrevice corrosion can not be prevented by increasing the Cr content andCRI value only. No crevice corrosion occurred in any of the valves whichwere made from a material comprising 18 to 25% of Cr, 15 to 25% of Ni,4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu and 0.1 to 0.3% of N and having aCRI value of at least 40.

The results given in Table 1 reveal that the fluid handling parts of theinvention have very high crevice corrosion resistance. TABLE 1 CreviceOverall Valve corro- evalua- No. Cr Ni Mo Cu N W V sion CRI tion Comp. 118.08 8.23 0.14 0.21 0.002 — — Occurred 18.7 X Exam- 2 17.67 11.87 2.540.19 0.0018 — — Occurred 27.88 X ple 3 19.33 13.25 3.55 0.22 0.0015 — —Occurred 33.58 X 4 19.02 0.1 1.99 0.05 0.001 — 0.12 Occurred 27.01 X 525.03 7.03 3.05 0.01 0.19 0.55 — Occurred 42.93 X Inven- 6 22.1 24.324.54 1.55 0.23 — — No 47.16 ⊚ tion 7 20.03 18.23 6.01 0.79 0.18 — — No49.47 ⊚ Exam- 8 20.04 19.11 5.53 1.09 0.13 — 0.34 No 46.06 ⊚ ple 9 24.0525.23 6.11 2.99 0.21 1.89 1.97 No 54.79 ⊚

As described above, the valve of the invention is usable with diminutionof crevice corrosion faults due to chemicals and foods, and hasdurability for use over a prolonged period of time without replacementor repairs of corrosion faults. The present invention is therefore ofimmense value.

INDUSTRIAL APPLICABILITY

The fluid handling parts of the invention have high crevice corrosionresistance, and are usable as couplings in piping systems and as valvesin fluid control apparatus and suitable especially for use inpharmaceutical or food manufacturing apparatus.

1. A valve, coupling or like fluid handling part for use in piping andfluid control devices which is composed of a plurality of components,the plurality of components including a specified metal member having acontact surface to be brought into contact with one of the othercomponents, the contact surface having an end exposed on an exterior ofthe fluid handling part, the fluid handling part being characterized inthat the specified metal member is made of an alloy comprising, in % byweight, 0.001 to 0.01% of C, up to 5% of Si, up to 2% of Mn, up to 0.03%of P, up to 100 ppm of S, up to 50 ppm of O, 18 to 25% of Cr, 15 to 25%of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu, 0.1 to 0.3% of N, and thebalance substantially Fe and other inevitable impurities, the alloyhaving a CRI (crevice corrosion resistance index) value in the range of40≦CRI≦55, as determined from the expression:CRI[=Cr]+4×[Mo]+30×[N] wherein the amounts of alloy components presentin combination in the alloy to ensure crevice corrosion resistance areexpressed in % by weight.
 2. A fluid handling part according to claim 1which is characterized in that the alloy further comprises up to 2% of Wand/or up to 2% of V.
 3. A fluid handling part according to claim 1 or 2which is a valve comprising a body, an actuator and a threaded member,at least one of the body, the actuator and the threaded member being thespecified metal member.
 4. A fluid handling part according to claim 1 or2 which is a diaphragm valve wherein a nonmetallic diaphragm is heldbetween a metal body and a metal bonnet, each of the body and the bonnetbeing the specified metal member.
 5. A fluid handling part according toclaim 1 or 2 which is a pipe coupling to be assembled by tightening up acap nut as screwed on an externally threaded portion provided on anouter periphery of a tubular coupling member, at least one of thecoupling member and the cap nut being the specified metal member.